1. Field of the Invention
The invention relates to a plant for producing metal melts, in particular steel melts from scrap, wherein the plant includes a direct-current electric arc furnace connected to a electricity supply system.
2. Related Art
The use of electric arc furnaces for producing metal melts, in particular for melting scrap, presupposes a electricity supply system of suitable strength, since during the operation of electric arc furnaces the load currents of an electric arc furnace may vary over a wide range, due to short-circuits caused, e.g. by falling-down scrap, sponge iron, etc. This entails effects on the electricity supply system, their most disagreeable manifestation being the so-called "flicker trouble". This phenomenon consists in changes in the luminous intensity of the lighting equipment powered by the electricity supply system of the electric arc furnace due to system voltage fluctuations (cf. Radex-Rundschau, issue 1/2, 1982, "Die elektrische und mechanische Auslegung des Elektrolichtbogenofens", "Electrical and Mechanical Dimensioning of the Electric Arc Furnace", the paper includes a summary in English!, pp. 724 through 756).
In order to avoid flickering it is known to raise the connection point of an electric arc furnace with a electricity supply system that also supplies the general public, the so-called PCC (Point of Common Coupling), to a higher voltage level--which is tantamount to a higher capacity level of the electricity supply system. Further it is known to provide for dynamic compensation in order to reduce effects on the electricity supply system with alternating-current and rotary-current electric arc furnaces (Radex-Rundschau, issue 2, 1984, "Elektrische Auslegung und Ausrustung von Lichtbogenofenanlagen" "Electrical lay-out and equipment of electric arc furnace installation", the paper includes a short introduction in English!, pp. 360 through 378). In the event that the system short-circuit capacity required to avoid flickering is not available even if the PCC is provided at the highest voltage level of the electricity supply system, such a flicker compensating apparatus for balancing the rapidly fluctuating reactive power demand of electric arc furnaces is indispensable.
All of these considerations have, however, been aimed at rotary-current electric arc furnaces. With a direct-current electric arc furnace the effects on the electricity supply system are different from those regarding rotary-current electric arc furnaces, due to the direct-current engineering. With direct-current electric arc furnaces it is, for instance, feasible to reduce the fluctuation in reactive power, and thus the flicker-effects are considerably reduced. It is known to avoid flicker-effects with a direct-current electric arc furnace by ensuring that the system short-circuit capacity is at least half the system short-circuit capacity calculated for an alternating- or rotary-current electric arc furnace of the same nominal power. However, this still presupposes a relatively high system short-circuit capacity and may entail difficulties, particularly in such cases where a PCC of the required capacity level is not available. In this case the difficulties were coped with by expanding the electricity supply system through the addition of overhead lines or by means of dynamic compensation equipment, or else the furnace throughput was markedly reduced and the deferred operation of a plurality of electric arc furnaces considered. It must, however, be said that all of these options are structurally complicated and expensive.